Titanium materials are light in weight, yet high in strength and excellent in corrosion resistance, so are being used even for the exhaust systems of automobiles. The combustion gas discharged from the engines of automobiles and motorcycles is collected at an exhaust manifold and discharged by an exhaust pipe from an exhaust outlet at the rear of a vehicle. An exhaust pipe is formed split into several segments to enable insertion of a catalyst and muffler in the middle. In the Description, the entire system from the exhaust manifold to the exhaust pipe and exhaust outlet will be called an “exhaust system”.
As the material of such an exhaust system, Japanese Unexamined Patent Publication No. 2001-234266 discloses an invention relating to a titanium alloy having both cold workability and high temperature strength. Further, as methods which form an oxidation prevention film to increase the oxidation resistance, an invention relating to coating an antioxidant including aluminum powder (see Japanese Unexamined Patent Publication No. 1-022404), an invention relating to coating Al particles, Si particles, or Al—Si alloy particles (see Japanese Unexamined Patent Publication No. 2004-115906), an invention relating to Al—Ti-based vapor deposition plating (see Japanese Unexamined Patent Publication No. 6-088208), an invention relating to a film containing Al and N (see Japanese Unexamined Patent Publication No. 9-256138), an invention relating to hot dip plating of a surface layer containing Al or Si (see Japanese Unexamined Patent Publication No. 2005-036311), and the like have been disclosed. Further, as a surface-treated titanium material superior in oxidation resistance, a titanium material formed with a fired coating layer of 5 μm or more comprised of particles comprised of pure Al or an Al alloy containing 10 at % or less of Si between which a compound comprised of a metal element M (where M is one or more types of Ti, Zr, Cr, Si, and Al) and C and/or O is filled (see Japanese Unexamined Patent Publication No. 2006-009115) is disclosed.